1. Field of the Invention
This invention is in the field of instruments which determine the distribution of particle fall velocities and their sizes by measuring the rate at which solid particles of different sizes fall through a liquid, the specific gravity of which is less than the specific gravity of the particles.
2. The Prior Art
The range of particle sizes in a material is often crucial to the use of the material. For example, the strength of concrete depends critically on the size distribution of the sand used; pigments must be ground to a certain maximum size to be useful in paints; the size distribution of the sand on a river or ocean bottom tells the hydraulic engineer much about currents in the river or ocean, and so forth.
Traditionally, a particle-size analysis is done by sieving a sample through a series of sieves of progressively finer mesh, which enables one to report that, say, 20% of the sample was retained on a 50 mesh screen, 30% of what passed through was retaining on a 140 mesh screen, and so forth. Graduated sieve analysis, however, is prone to error as the finer mesh screens tend to clog. The sieve data are necessarily discontinuous, being dependent on the number of screens and the graduations of mesh used. It is a slow, tedious, labor-intensive process.
Improvements over the graduated sieve method involve determining the time it takes for particles to fall through a column of liquid, typically water. Assuming that the particles are of uniform specific gravity greater than the specific gravity of the liquid, a large particle will fall faster than a small one. It will do so because the drag force and the force of gravity acting on, for example, a sphere, are balanced during the settling process. The drag force is proportional to the product of the square of the diameter of the sphere and the square of the fall velocity; while the force of gravity is proportional to the cube of the diameter. Therefore, the fall velocity if proportional to the square root of the diameter, meaning that a large particle falls faster than a small one.
Consider a settling tube with a sensitive pressure sensor near its bottom. Fill it with water, and add a sample of particles. The sensor will sense a pressure comprising three components:
(a) the pressure equivalent to the initial pressure head of water above the sensor;
(b) the additional pressure representing the volume of water displaced by the sample, and possibly added with the sample;
(c) The additional pressure caused by the apparent weight of the sample in water.
As particles settle past the sensor, pressure component (c) decreases progressively until, after all of the particles have settled to the bottom, it becomes zero. The time record of the changing process while the particles are falling is a measure of the time of fall, and thereby of the changing sizes of the particles as they settle past the sensor. In principle, the rate of fall of the particles, and their weight and volume, and therefore the specific gravity of the particles, can be extracted from (a), (b), and (c).
The actual situation is not quite that straight-forward. The difficulty is that components (b) and (c) are a tiny fraction of component (a). If it is to provide sufficient sensitivity to detect component (c), the sensor must somehow be set to zero when the settling tube is filled.
Although many configurations have been proposed to this end, none has yet provided for the collection and use of all the information embedded in (a), (b), and (c). An example of the prior art is the design taught by Hartman, in U.S. Pat. No. 3,788,146.
The Hartman patent provides a settling tube with a "compensation pipe 2" parallel to it, and connected to the settling tube by a sensitive pressure transducer (Hartman calls it a "pressure difference pick-up") at its bottom end. The transducer measures the differential pressure between the settling tube and the compensation pipe at a point just above the bottom of the settling tube. In one configuration (Hartman's FIG. 2), just below the top of the settling tube a "connecting pipe" connects the settling tube and the compensation pipe so that the water level is the same in each. Hartman provides a "throttle", not a cut-off valve, in the connecting pipe; the connecting pipe remains open throughout the measurement. The fact that Hartman's connecting pipe is not sealed off during the measurement process is crucial to understanding why the sediment analyzer of this invention is a major improvement to the art.
When the operator adds his sample to Hartman's settling tube, the liquid level in the tube rises, proportionately to the water displaced by the sample, and the transducer records this increment. At the instant of adding the sample, the pressure increment at the pressure transducer is further enhanced by the increased density of the fluid in the settling tube, a consequence of the sample suspended in it. At this instant, Hartman's transducer records components (b) and (c) as defined above. But flow th the throttled "connecting pipe" into the compensation pipe quickly restores an equal level in the settling tube and in the compensation pipe. Information about the weight of the sample and its volume is effectively erased when the two liquid levels are equalized. As a consequence, each sample must be carefully weighed before each test, a requirement which can become burdensome when hundreds of samples are to be tested.
A need therefore exists for a liquid column particle size measuring instrument which will povide analyses of particle size distributions in real time without the requirement that each sample be individually weighed.